In recent years, it has been common for electronic devices such as mobile phones, tablet computers, digital cameras, and video game equipments, to have an audio function. A sound emitter such as a speaker and/or a sound receiver such as a microphone is placed as an audio part inside a housing of an electronic device having an audio function. An opening is usually provided in the housing of the electronic device at an appropriate position relative to the audio part. Sound is transmitted between the outside of the electronic device and the audio part through this opening.
Obviously, entry of water into the housing of the electronic device must be prevented; however, the above-described opening for sound transmission may constitute a route that allows water to easily enter the housing. Especially for portable electronic devices, the risk of water entry is high because they are often exposed to rain or water used in daily life and because the orientation of the opening cannot be fixed at a specific orientation that allows the avoidance of water (for example, a downward orientation for which rain is less likely to come into the housing). Accordingly, a waterproof sound-transmitting membrane that allows sound to be transmitted between the audio part and the outside of the housing and that prevents water from entering the housing from outside through the opening is placed so as to cover the opening.
An example of the waterproof sound-transmitting membrane is a stretched porous membrane having a structure in which a huge number of pores formed by stretching are distributed. Patent Literature 1 discloses a waterproof sound-transmitting membrane including a stretched porous membrane of polytetrafluoroethylene (PTFE) or ultrahigh molecular weight polyethylene. Another example of the waterproof sound-transmitting membrane is a non-porous resin film having formed therein a plurality of through holes extending through the thickness of the film (see Patent Literature 2).